Fluidized bed filters are used widely in aquaculture, water purification, and wastewater treatment. These filters can also be used in bioremediation. Under pneumatic pressure, fluidized bed filters are used to filter and treat gases, to size and separate particles, to mix powdered components, and to coat particulate substrate. The fluidized bed filters used in aquaculture are specialized biological filters which are used to remove waste products from the recirculating culture system.
Aquaculture is a term used generally to describe a variety of methods and systems for producing plants and animals in water environments. Thus, aquaculture can involve not only the cultivation of fish but also includes the cultivation of many aquatic plants and a variety of aquatic animals in addition to fish. Alligators, crawfish, frogs and seaweed are routinely cultivated by aquaculture. As the demand for healthful and exotic foods rises, aquaculture continues to expand. New species are being cultivated and advances are being made toward the increase in production of aquaculture systems. Additionally, the popularity of fresh and saltwater aquariums has enjoyed a resurgence further adding to the demand for better aquaculture systems. As used herein, the term "aquaculture" refers not only to systems for the commercial production of plants and animals, but also to consumer systems such as fresh and saltwater aquariums.
Providing for artificial cultivation of fish and other animals in populations far more dense than in nature, is a difficult prospect. Adequate food and oxygen must be supplied to the population. Further, waste must be continuously removed from the water. To maintain a supply of food and oxygen to the plant or animal population, water can be continuously pumped from a suitable source through an aquaculture system, then discharged. This, however, requires great quantities of water to maintain the system. Unfortunately, water is often a limiting factor in aquaculture. Also, the wastewater must be treated and disposed of properly. Therefore, semi-closed aquaculture systems have been developed in which water is recirculated within the system (Bullock et al, 1993). Only small amounts of new water are added periodically in a semi-closed system. Oxygen and nutrients are added to the recirculating water and waste is removed through appropriate filtration and treatment systems.
Waste can be removed from a semi-closed system by utilizing any or all of three types of "filtering" means: mechanical, chemical, and biological. Each type of filter has certain advantages with regard to the type of waste it removes most efficiently. Therefore, the filter types are often used in succession to effectively clean the recirculated water.
Particulate matter can be removed by a mechanical type filter which physically removes solid waste from the water. Mechanical filtration is often the initial step in a filtration process because particulate waste eventually breaks down and can interfere with the operation of other filters. Chemical filters typically absorb or adsorb dissolved organic compounds from the water and most often comprise an inert adsorbent such as activated charcoal.
Biological filters can remove ammonia and other metabolites from the water. Ammonia waste is of particular concern in fish aquaculture because ammonia is generated by the fish population but is toxic to the fish. Therefore, ammonia must be effectively removed from the system. In nature, ammonia waste is normally removed from water through natural processes by "ammonia-eating" bacteria. These bacteria convert ammonia to other nitrogen-containing compounds through their normal metabolic activity. Also, because of the large volume of water in nature, the concentration of ammonia is generally quite low. Fish populations in aquaculture systems are far more dense than in nature; therefore, ammonia cannot generally be adequately removed from these systems by the bacteria which naturally exist in the water. Biological filters are often used, therefore, to assist in the ammonia removal process. Biological filters can comprise bacteria which convert ammonia to other compounds. These bacteria colonize the filter media and use, as a food source, nutrients in the water. Ammonia is a primary food source for the colonizing bacteria and, by the bacterial action, ammonia waste is removed from the water.
A primary function of the biological filters is to concentrate the ammonia-degrading bacteria and to allow them to proliferate. Typically, ammonia is first metabolized to nitrite by Nitrosomas sp. bacteria. Nitrites are also toxic to aquatic life; however, different bacteria convert these nitrites to nitrates. For example, nitrites are converted to nitrates by Nitrobacter sp. bacteria. The nitrates are then converted to harmless, inert nitrogen gas by plants or anaerobic bacteria. Each of the bacterial species necessary to remove the ammonia from the system is present naturally in the water. The concentrated, enriched population of bacteria in a biological filter can efficiently and effectively remove ammonia generated by even dense populations of fish.
Currently, biological filters used in aquaculture include fluidized sand filters (Malone and Burden, 1988). A fluidized sand filter typically comprises an open chamber approximately half-filled with sand. An inlet port is positioned in the center of the bottom of the chamber. The sand within the chamber is colonized by bacteria. Waste water is pumped into the system through the inlet port. The force generated by the flow of the water expands, or "fluidizes," the sand within the water-filled chamber. Water contacting the sand colonized by the bacteria is cleansed of ammonia by the action of the bacteria. Treated water leaves the system through an outlet port near the top of the open chamber.
Some of the advantages of using a fluidized bed for biological filtration are as follows:
1. There is no mechanical particle capture ability in these filters, which means that they do not clog during use. This is a rather significant problem with other filtration systems. Since the media bed is fluidized and constantly moving, any particles that enter the filter ultimately pass through without becoming lodged.
2. Since a biological filter depends upon bacteria to function, it is very important to have as much surface area available for growth as possible. Fluidized bed filters have very large amounts of surface area, due to the fact that they use a fine grade of media which has high surface area for its volume.
3. What makes a fluidized bed filter especially unique is the fact that it can fully utilize all of the available media surface area, since the media bed is in constant motion. This allows the incoming water to be thoroughly mixed with the bacteria on the medium, instead of passing around it as in a fixed media filter. This method of ammonia removal is highly efficient.
Despite the advantages of fluidized bed filters for aquaculture, there are still significant drawbacks to their use. For example, should flow of the water cease, the filter medium, by the force of gravity, settles to the bottom of the chamber. Existing filters utilize a variety of designs to try to prevent the media from settling into the entry tube. In one design, a perforate disk overlays the inlet port of the fluidized sand filter, pea gravel is layered on top of the disk. The pea gravel layer and perforated disk help to prevent the filter medium from settling into the inlet port. However, settled filter medium can still pack the port and the bottom of the chamber and can obstruct the flow of water. Obstructions are difficult to clear and often require significant water pressure to blast settled filter medium free and refluidize a system. It is often necessary to completely replace choked filters. It is therefore essential that these fluidized sand filters are monitored continuously to prevent shut down.
The drawbacks of the sand biofilters are common to all fluidized bed filters; therefore, a simple, efficient fluidized filter which requires little maintenance and does not choke upon the loss of pressure or flow would greatly improve any fluidized filtration system.